Polyolefins such as polyethylene (PE) and polypropylene (PP) may be used to manufacture a varied range of articles, including films, molded products, foams, and the like. Polyolefins may have characteristics such as high processability, low production cost, flexibility, low density and recycling possibility. However, physical and chemical properties of polyolefin compositions may exhibit varied responses depending on a number of factors such as molecular weight, distribution of molecular weights, content and distribution of comonomer (or comonomers), method of processing, and the like.
Methods of manufacturing may utilize polyolefin's limited inter- and intra-molecular interactions, capitalizing on the high degree of freedom in the polymer to form different microstructures, and to modify the polymer to provide varied uses in a number of technical markets. However, polyolefin materials may have a number of limitations, which can restrict application such as susceptibility to deformation and degradation in the presence of some chemical agents, and low barrier properties to various gases and a number of volatile organic compounds (VOC). Property limitations may hinder the use of polyolefin materials in the production of articles requiring low permeability to gases and solvents, such as packaging for food products, chemicals, agrochemicals, fuel tanks, water and gas pipes, and geomembranes, for example.
While polyolefins are utilized in industrial applications because of favorable characteristics such as high processability, low production cost, flexibility, low density, and ease of recycling, polyolefin compositions may have physical limitations, such as susceptibility to environmental stress cracking (ESC) and accelerated slow crack growth (SCG). Which may occur below the yield strength limit of the material when subjected to long-term mechanical stress. Polyolefin materials may also exhibit sensitivity to certain groups of chemical substances, which can lead to deformation and degradation. As a result, chemical sensitivities and physical limitations may limit the success in the replacement of other industry standard materials, such as steel and glass, with polyolefin materials because the material durability is insufficient to prevent chemical damage and spillage.
Conventionally, methods of altering the chemical nature of the polymer composition may include modifying the polymer synthesis technique or the inclusion of one or more comonomers. However, modifying the polyolefin may also result in undesirable side effects. By way of illustration, increasing the molecular weight of a polyolefin may produce changes in the SCG and ESC, but can also increase viscosity, which may limit the processability and moldability of the polymer composition.
Other strategies may include inclusion of a comonomer and/or blending polyolefins with other polymer classes and additives to confer various physical and chemical attributes. For example, polyolefins may be copolymerized with alpha-olefins having a lower elastic modulus, which results in a considerable increase in environmental stress cracking resistance (ESCR) and resistance to impact but adversely affects the stiffness of the polymer. However, the use of alpha-olefins may have limited effectiveness because, while the incorporation of alpha-olefin comonomers must occur in the high molecular weight fraction in order to affect ESC and impact resistance, many popular catalyst systems have a low probability of inserting alpha-olefins in the high molecular weight fraction, an important factor in forming “tie molecules” between the chains of the surrounding polyolefin that are responsible for transferring stress between the crystalline regions and, consequently, responsible for important mechanical properties. The end result is the production of a polymer composition having reduced structural stiffness. It is also noted that, while advances have developed catalysts that increase the likelihood of displacing the incorporation of a comonomer to the highest molecular weight range, and that multiple reactors may be used to address these limitations, such modifications are expensive alternatives and not wholly effective in balancing resistance to impact and ESC without negatively affecting stiffness.
Polymer modification by blending may vary the chemical nature of the composition, resulting in changes to the overall physical properties of the material. Material changes introduced by polymer blending may be unpredictable, however, and, depending on the nature of the polymers and additives incorporated, the resulting changes may be uneven and some material attributes may be enhanced while others exhibit notable deficits. The incorporation of a second phase into the matrix polymer, which generally has a different chemical nature, may increase the resistance to impact and ESC resistance in some cases. However, like the copolymerization strategy, polymer blends are often accompanied by a marked loss in stiffness, because the blended materials may have lower elastic modulus than the matrix polyolefin.
Accordingly, were exists a continuing need for developments in polyolefin compositions to have increases in environmental stress cracking resistance while balancing the mechanical properties of the polymer. There also exists a continuing need for polyolefin compositions having good barrier properties to various gases and volatile organic compounds.